The present invention relates in general to intake manifolds for internal combustion engines. More in particular, the present invention relates to intake manifolds of the high performance type.
A carburetor internal combustion engine employs an inlet manifold to distribute a fuel-air mixture produced by the carburetor into the cylinders of the engine. The mixture is drawn into the combustion chambers of the engine by a vacuum created there by piston movement during the "suction stroke" of each cylinder. The amount of work done by the engine to produce the vacuum and draw the fuel-air mixture into the combustion chambers forms a part of the engine's "pumping-friction" work.
In a V-8 engine there are typically eight inlet ports for the passage of the fuel-air mixture into the eight combustion chambers of the engine. An inlet manifold for a V-8 engine communicates the carburetor with the engine's inlet ports through "runners". A runner is a duct or passageway. When two of these "ducts" are side-by-side the combination of the two is often called "a runner" with each duct called "a leg." Usage also permits that each of the side-by-side ducts be called a runner and this meaning will be employed throughout this specification. In any event, individual runners between each of an engine's inlet ports and a plenum of the manifold located immediately below the carburetor are known.
The induction of fuel-air mixtures into an internal combustion engine is an extremely complicated phenomenon and has given rise to several conflicting problems.
One of the most important problems is pumping-friction work. As previously mentioned, a fuel-air mixture is inducted into an internal combustion engine through the manifold. The engine acts as a pump when it produces the vacuum responsible for the pressure drop through the manifold between atmosphere and the combustion chambers, which pressure drop constitutes the driving force acting on the fuel-air mixture. Obviously this pumping requires power. Power lost to flow losses of the mixture through the manifold reduces the engine's output and its efficiency. As a consequence of this, one aspect of good manifold design is to provide minimum losses because of flow phenomena.
Another problem in manifold design is the effect of the pressure history of individual cylinders on other cylinders. Pressure pulses, both positive and negative with respect to atmosphere, travel up and down the runners of a manifold and are generated from such constantly recurring events as inlet valve openings. While a pressure pulse phenomenon can sometimes be used to advantage in augmenting the driving force acting on the mixture during its induction into the cylinders, the phenomenon can actually reduce the driving force unless the phase relationship of the pressure pulses is just right. Pressure pulses can also lead to a problem known in this art as "standoff." Standoff is a condition where fuel-air mixture is forced back through a manifold and carburetor to atmosphere because of a pressure condition existing in the manifold. Standoff occurs at well-defined engine speeds for a particular engine-manifold-carburetor combination. Standoff manifests itself as a cloud of gasoline vapor and droplets over the carburetor.
Another problem in good manifold design is to provide a uniform fuel-to-air mixture in each of the cylinders it supplies. Carbureted fuel is a mixture of vaporized fuel, atomized fuel and liquid fuel. Liquid fuel travels along the walls of a runner towards an inlet port under the influence of the gaseous mixture passing through the runner above it and gravity. In practice, this liquid component of the fuel charge has made it extremely difficult to keep fuel-to-air ratios uniform to each of the cylinders of an engine. Atomized fuel is not truly a vapor but is instead very fine particles of liquid. Atomized fuel is carried in suspension by the air stream between the carburetor and the cylinders. Because the particles of atomized fuel are heavier than their carrying air stream there is a tendency for them to come out of suspension when the fuel-air mixture turns a corner. This is because the vapor has a tendency to go straight while the gas wants to turn the corner. When the atomized fuel comes out of suspension, the problem of keeping the fuel-air ratio the same for all cylinders is, of course, aggravated.
In an effort to maintain atomized fuel in suspension in the mixture stream, it has been the practice to increase the kinetic energy of the atomized fuel by increasing the velocity of the mixture through the runners. The velocity of the mixture is increased by reducing the cross-sectional area of the runners. But the approach of increasing atomized fuel kinetic energy obviously runs into problems when corners or bends in the runners are required, for the fuel particles will strike the outside wall of the bend and come out of suspension.
One of the most popular manifolds produced in this country is the so-called two-plane, over and under, 180.degree. manifold. This manifold has been a standard for most American production V-8 engines for use with a single, standard four-barrel carburetor for some time. The manifold has runners disposed in a relatively complex pattern. The design of the manifold attempts to minimize the problems of efficient fuel distribution, the adverse effects of pressure interference of one cylinder on another cylinder, and standoff. But the two-plane, 180.degree. manifold is a compromise. It uses a twisting, tortuous path in each of the runners which results in excellent control of inter-cylinder interference and standoff but produces poor air-to-fuel ratio uniformity between cylinders and high "pumping-friction" work because of high flow losses. Another problem with the tortuous paths of the runners in the two-plane manifold is that inter-cylinder fuel-to-air ratios vary over a wide range resulting in a compromise in carbureting an engine which produces less than optimum emissions and performance.